1. Field of the Invention
This invention relates to a test method and an apparatus for a reticle or mask pattern used in fabricating a semiconductor integrated circuit, more particularly to a method in which a real image of a reticle or mask pattern formed by an optical means is compared with a pattern reproduced by a graphical means from an original design data, and thereby the reticle or mask is inspected to determine if it is acceptable or not. More precisely, this invention relates to a test method and an apparatus for the reticle or mask pattern, which is formed with a slight modification by reducing or magnifying the original design pattern in a similar shape.
2. Description of the Prior Art
In fabricating steps of a semiconductor integrated circuit, a plurality of the same patterns are formed on each chip area on a wafer by a photolithography method, and such steps include a formation of an insulating layer, a diffusion or implantation of impurities, a formation of metal layer and the like. In the photolithography technology, the reticle or mask is used to expose a resist layer coated on a wafer corresponding to the pattern to be formed thereon. The reticle has a pattern having a size of 5 to 10 times as large as the real pattern required on the wafer. Its pattern is formed utilizing a pattern generator, wherein the original design data is installed. On the other hand, the mask has a pattern having the same size as the real pattern required on the wafer, and the mask pattern is formed using a reticle.
In an exposure step for the resist layer on the wafer using a reticle, the reticle pattern is directly projected on the wafer with a reduction ratio of 1/5 to 1/10 by a step and repeat process. In using a mask, on the other hand, the mask has plurality of chip patterns same as those required on the wafer, therefore, the mask is placed directly on the wafer and the resist layer is exposed under this mask.
The pattern formed on the reticle is required to be an exact replica of the original pattern of the design data, in other words, a magnified pattern 5 to 10 times as large as the original in a similar shape. However, in forming the reticle pattern, the reticle is subject to an exposure step using a pattern generator, a developing step and the like. On the other hand in fabricating a mask, the mask is subject to exposure steps with a reduction ratio by step and repeat process using the reticle. Therefore, in fabricating process of the reticle or mask, dimensional errors of patterns, growth of flaws, adhesion of dusts and etc. can not be avoided.
Therefore, inspection of patterns formed on a reticle or mask is an important factor. In testing these patterns, a comparison method has been put to practical application, wherein a real image pattern of reticle or mask is formed by optical and electrical means, and is compared with a standard pattern. As the standard pattern, the reproduced pattern from the original design data is used very often recently.
The above comparison method is disclosed in the following U.S. patents.
U.S. Pat. Nos. 4,603,974, Aug. 5, 1986, issued to S. Matsui; 4,641,353, Feb. 3, 1987, issued to K. Kobayashi; 4,673,816, Jun. 16, 1987, issued to S. Matsui and K. Kobayashi; Appl. No. 620,089, Jun. 13, 1984, by S. Matsui and K. Kobayashi.
The design data of the patterns is generally stored in a magnetic tape using a specific format. Generally, the data in the magnetic tape can not be directly used for the pattern generator in reticle fabrication. Furthermore, the above data also can not be used in a test apparatus for reproducing the original pattern which is compared with a real image pattern of reticle or mask. Therefore the design data should be once subjected to a format conversion using a conversion program for the pattern generator or the test apparatus respectively.
In case of forming a pattern on the reticle, for example, a small area of the reticle is sequentially exposed using pattern generator by moving a reticle stepwise, which is loaded on a stage thereof, and the reticle surface is swept in such a way of moving in a X-direction, shifting Y-position intermittently. In this case, the original design data is to be converted into the format conforming with the specifications of the pattern generator.
The pattern generator has a specification of so-called a field which defines an unit area, during which the reticle substrate is exposed while it is stationary. Generally, the pattern generator has an unit field of 100.times.100 .mu.m.sup.2 to 500.times.500 .mu.m.sup.2, and the reticle is exposed one by one at every field until the entire surface of the reticle is exposed.
Similarly, the test apparatus for a reticle or mask pattern has a prescribed unit field and it necessitates a conversion of the original design data matched with the specific format applicable for the test apparatus.
When different processes are expected to be applied for the specific step in fabricating a semiconductor integrated circuit, the test for reticle pattern becomes more intricate. Because the plural reticles, each having a slight modification in pattern dimensions by reducing or magnifying the original pattern with similar shape are often fabricated. Such a slight modification of reticle pattern is achieved only by changing the conditions of an exposure or chemical treatments.
In another case, it becomes necessary to fabricate a reticle having a slight modification of pattern in order to fabricate a semiconductor integrated circuit which has a slightly different characteristic from the original type. In this case, the slight modification of reticle pattern is also performed by changing conditions of an exposure or processes of chemical treatments.
All reticles having a slight modifications as described above are fabricated using the same design data for the pattern generator, however they go through the slight different process such as exposure or chemical treatment and the like. Accordingly, the format conversion of the original design data for the pattern generator is carried out only once, however, plural kinds of reticles each having a slight modification of the pattern are fabricated from the same design data.
However, in testing these reticles, format conversions of the design data corresponding to each pattern modification are necessary in the prior art. It takes a lot of operating hours of a computer and man-hours.
With regard to a very large scale integrated circuit, format conversion of the original design data is carried out by operating a large-scale computer for long hours. Modification of design data and format conversion are necessary for each modified pattern. Other problems concerned with the above conversion are the maintenance of conversion programs and safe keeping of converted pattern data, because many kinds of reticles or masks having a different pattern size are needed and fabricated even for one original type of an integrated circuit. Therefore, a reliable keeping of these data are important.